@Preston wrote:
Stupid question... but what's a good definition of "group" in terms of flashes? That has me confused.
Well, it's a group of flashes, although a group can have a single flash in it. It's a way to organize sets of flashes for adjustment. Every flash in a group will have the same power settings. But if you're only using three flashes, then you'd probably give each flash it's own group (although IIRC Canon has some limitations as to what you can do with Group C in eTTL).
In eTTL mode you set power ratios. For example, you tell it that you want Group A to be 2 times as powerful (1 stop) as Group B and 4 times (2 stops) Group C. A would be your Key light, B your fill, and C your background light (just as a general example, there are no rules). It would look like this (A:B:C) - 4:2:1 The camera then determines how much light it thinks it needs and calculates how strong to make each group given that ratio.
In manual mode you just adjust the power level of the entire group, 1/1 being full power of any flash in that group. So say you decide that your key light needs to be 1/4 power. You set Group A (a single key light) to 1/4 power, your second flash gets assigned to Group B at 1/8 (one stop less) for fill, and then you have a couple accent lights assigned to Group C that you set to 1/16 (two stops lower than A) for background lighting. These are just example numbers to mimic what was set in the eTTL example above. 4:2:1 ratio.
